This application claims the priority of Japanese Patent Applications Nos. 8-93330, 8-93331 and 8-93332 filed on Mar. 22, 1996 which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light source device for an electronic endoscope and, more particularly, to a structure for eliminating the noise produced in a light source device which controls the lighting operation of a xenon lamp.
2. Description of the Related Art
Electronic endoscopes are intraperitoneal mirrors for picking up an image of a body cavity as an object of observation by a CCD, which is a charged coupled device disposed at the end portion of an electronic endoscope. Light for irradiating the body cavity is supplied through a light guide disposed from a light source device to the end portion.
FIG. 10 shows the structure of an example of a conventional light source device. A box body 1 is provided therein with a lamp house 3 accommodating a xenon lamp 2, and a lighting circuit 5 connected to the xenon lamp 2 through an igniter 4. The lighting circuit 5 is fixed onto a substrate 7 supported by an L-shaped aluminum support 6, which, in turn, is anchored to a bottom plate 1A of the box body 1 through an insulating plate 8 such as a bakelite sheet.
The insulating plate 8 maintains a withstand voltage of not less than 4 kV between the lighting circuit 5 (e.g., primary) and the box body 1 so as to secure the safety of the box body 1. A connector receptacle 9 for receiving a light guide connector of an electronic endoscope is provided on the front panel of the box body 1, as shown in FIG. 10.
FIG. 11 shows the waveforms of the signals in the lighting circuit 5. When a switching operation is executed in accordance with the control pulses shown in Fig. 11(A), a switching current having a high frequency such as about 110 kHz is formed, as shown in Fig. 11(B). The switching current is supplied to the igniter 4 shown in FIG. 10, whereby the igniter 4 discharges, and the xenon lamp 2 in the lamp house 3 is lighted. The light of the xenon lamp 2 is supplied to the light guide of the connector of the electronic endoscope which is fitted into the connector receptacle 9, and introduced to the end portion through the light guide, thereby enabling the image pick-up operation of the CCD.
According to the above-described light source device, however, the switching current of the lighting circuit 5 produces a high-frequency noise, which exerts a deleterious influence on an image or the like. FIG. 12 shows the noise produced in the lighting circuit 5. As is clear from FIG. 12, since the switching current has a frequency of about 110 kHz, a noise signal having a frequency of an integral multiple of 110 kHz is produced. In addition, since the switching current is comparatively large, the noise maintains a certain degree of level until a high frequency range such as several ten MHz. Since the lighting circuit 5 of the light source device is not in proximity to the processor circuit for processing video signals, no measure against the noise produced in the lighting circuit 5 has been taken in the present state.
Furthermore, a high-frequency noise is also produced due to the discharging operation of the xenon lamp 2, and exerts a deleterious influence on an image or the like. To prevent this, the xenon lamp 2 is accommodated in the metal lamp house 3, which is electrically connected to the box body 1, and a noise current is introduced to the ground so as to prevent the noise produced from the xenon lamp 2 from radiating outward.
However, the electrical connection between the lamp house 3 and the boxy boy 1 is not so good as to certainly maintain the lamp house 3 and the box body 1 at the same potential and to securely eliminate noise. More specifically, the lamp house 3 is connected to the box body 1 by fixing screws and conductive wires on the underside of the lamp house 3. Such a connecting method leaves only a small contact area, which makes it impossible to smoothly introduce the noise current to the ground of the box body.
In addition, the noise component induced to the upper surface of the lamp house 3 is sometimes radiated therefrom before it is led to the ground beneath the lamp house 3, so that it is impossible to completely eliminate such noise.
Furthermore, since a noise current is transmitted to each member through a conductive wire which connects the lighting circuit 5 to the igniter 4 and a conductive wire which connects the igniter 4 to the xenon lamp 2, a noise signal is sometimes produced from such a member in the device.